This invention generally relates to the dissolving pulp cooking and particularly with pre-hydrolysis and Kraft cooking.
A pre-hydrolysis of wood chips for the removal of hemicelluloses prior to Kraft cooking may be required in one method of producing pulps having a high alpha cellulose content (e.g., greater than 94% alpha cellulose). Pulps with such high alpha cellulose content are typically referred to as dissolving pulps and may be used in the production of rayon, acetate and other products.
A conventional method of carrying out the pre-hydrolysis of wood chips involves contacting the wood chips with a liquid (water) environment at temperatures in the range of 150-170° C. One disadvantage to the liquid surrounding the wood chips is that there is an opportunity for the byproducts of the pre-hydrolysis reaction to diffuse from the wood chips into the surrounding liquor. These byproducts consist of various sugars such as xylose, furfural, arabinose, mannose, galactose, as well as acetic acid, other organic acids and lignin fragments.
The released acids can lower the pH of the mixture to the range of 3.3-3.7, such lowered pH further driving the hydrolysis reaction. The hemicelluloses are generally present as monomers and oligomers and more complex molecules.
Under the pH and temperature conditions of the reaction, and particularly higher temperatures, some of the complex molecules and lignin fragments undergo condensation reactions and combine into higher molecular weight molecules (pseudo-lignin, lignin and other condensation reaction byproducts) which are susceptible to precipitation on the surface of the equipment and the wood chips.
The precipitation onto the surface of the equipment, particularly liquor extraction screens, can make the operation unstable and can force premature stoppage of the process or switching to another mode of operation for cleaning of the equipment.
Methods of pre-hydrolysis and/or other known apparatuses are described, for example, in U.S. Pat. No. 6,280,569 to Sheerer; U.S. Pat. No. 5,985,096 to Marcoccia et al.; U.S. Pat. No. 5,676,795 to Wizani et al.; U.S. Pat. No. 5,589,033 to Tikka et al.; U.S. Pat. No. 5,454,490 to Johanson; U.S. Pat. No. 4,028,171 to Richter; U.S. Pat. No. 3,413,189 to Backlund; U.S. Pat. No. 3,380,883 to Richter et al.; U.S. Pat. No. 2,858,211 to Durant et al.; U.S. Patent App. Pub. No. 2011/0180061 to Bolles et al.; and U.S. Patent App. No. 61/445,253 to Leavitt et al. (filed Feb. 22, 2011). In addition, related techniques and apparatuses are described in Leschinsky et al., Formation of Insoluble Components During Autohydrolysis of Eucalyptus Globulus, Lenzinger Berichte 87 (2009) 16-25; Sixta, Multistage Kraft Pulping, Handbook of Pulp, p 325-365 (2006); Rydholm, Chemical Pulping—Multistage Processes, Pulping Processes, p. 655-671 (1965); and Rydholm, Continuous Prehydrolysis-Kraft Cooking, Continuous Pulping Processes, p. 105-120 (1970)